1. Field of the Invention
The present invention relates to a refrigerating cycle apparatus provided two compressors with and condenser and evaporator.
2. Description of the Prior Art
The refrigerating cycle apparatus provided with two or more compressors with respect to a condenser and evaporator so that one or a plurality of compressors are shut down for carrying out load control has hitherto been disclosed in the Japanese Patent Publication No. 54-20020 (1979) and the Japanese Patent Application Laid-Open No. 56-27868 (1981).
FIG. 1 is a system diagram of construction of the conventional refrigerating cycle apparatus in which two compressors 1 and 2 are provided.
In the drawing, the first compressor 1 has a suction pipe 7 from which a suction pipe 8 for the second compressor 2 is branched. The first and second compressors 1 and 2 are connected at the discharge sides thereof through check valves 11 and 10 to the inlet side of a condenser 3 for liquefying a refrigerant, the condenser 3 being connected at the outlet side thereof through a thermal expansion valve 4 to an inlet side of an evaporator 5 for vaporizing the refrigerant. In addition, a temperature sensing cylinder 41 for the thermal expansion valve 4 is provided at an outlet pipeline of the evaporator 5. The outlet pipeline of the evaporator 5 is also connected to the inlet side of an accumulator 6 for removing liquid drops from the refrigerant gas, the accumulator 6 being connected to the outlet side of the suction pipe of the first compressor 1.
Shells of the first compressor 1 and that of the second compressor 2 are connected at the lower portions with each other by an equalizer pipe 9 in order to equalize the internal oil amounts in the shells.
The conventional refrigerating cycle apparatus constructed as above-mentioned discharges a high temperature and high pressure refrigerant from the first and the second compressors 1 and 2, so that the refrigerant is fed to the condenser 3 via the check valves 11 and 10 and liquified in the condenser 3. Next, the pressure of the liquified refrigerant, that is, liquid refrigerant, is reduced by the thermal expansion valve 4 and thereafter revaporized by the evaporator 5 so as to be drawn again into the first and the second compressors 1 and 2 via the accumulator 6, thus forming the refrigerating cycle provided with two compressors 1 and 2 with respect to condenser 3 and evaporator 5.
In this refrigerating cycle, the suction pipe 7 for the first compressor 1 has a riser piping 7a through which the refrigerant rises from the accumulator 6 and a descending piping 7b through which the refrigerant, having passed the riser piping 7a, descends, the suction pipe 8 for the second compressor 2 being connected with an intermediate portion of the descending piping 7b in a manner that the utmost end of suction pipe 8 is plunged and projected inside of the suction pipe 7. Therefore, oil drops through the descending piping 7b by the gravity and is not drawn into the suction pipe 8 for the second compressor 2. Hence, although the refrigerant and the oil are drawn into the first compressor 1, only the refrigerant is drawn into the second compressor 2.
On the other hand, the suction pipe 7 for the first compressor 1 and that 8 for the second compressor 2 are selected in diameter so that pressure P.sub.2 in the shell of the second compressor 2 is lower than that P.sub.1 in the shell of the first compressor 1. Concretely, the suction pipe 8 for the second compressor 2 is longer than the suction pipe 7 for the first compressor 1 and smaller in an inner diameter than that of the descending piping 7b from which the suction pipe 8 is branched. Hence, resistance of the suction pipe 8 becomes larger than that of the descending piping 7b, whereby the pressure P.sub.2 in the shell of the second compressor 2 is lower than that P.sub.1 in the shell of the first compressor 1.
Thus, even when the oil is to be drawn only in the first compressor 1, the difference in pressure feeds a considerable amount of oil into the second compressor 2 through the equalizer pipe 9.
Such oil-feed operation occurs not only when the two compressors 1 and 2 are simultaneously operated but also when the first compressor 1 is shut down and the second compressor 2 is operated in unloading condition. In other words, the oil discharged from the accumulator 6, which drops by gravity through the descending piping 7b of the suction pipe 7 for the first compressor 1, enters directly into the first compressor 1, the refrigerant being drawn into the second compressor 2 through the suction pipe 8.
The pressure P.sub.1 in the shell of the first compressor 1 under shutdown, as above-mentioned, is higher than the pressure P.sub.2 in the shell of the second compressor 2 under operation (P.sub.1 &gt;P.sub.2), whereby the oil drawn into the first compressor 1 under shut-down transfers at part above the equalizer pipe 9 to the second compressor 2 under operation, through the equalizer pipe 9, when the oil level exceeds the height of the equalizer pipe 9. Accordingly, the oil level in the first compressor 1 under shutdown is substantially level with the height of equalizer pipe 9, resulting in that the oil level in the second compressor under operation becomes fairly high.
Furthermore, in the aforesaid conventional refrigerating cycle apparatus, when the second compressor 2 is individually operated on condition of flood back wherein gas refrigerant including liquid refrigerant is compressed, the liquid refrigerant is separated from the gas refrigerant and drops by the gravity through the descending piping 7b of the suction pipe 7 to thereby flow into the first compressor 1, thus presenting the phenomenon of the socalled excessive accumulation of the liquid refrigerant in the crank case during the off-cycle. Hence, in the bottom portion of the shell of the first compressor 1 are gradually collected the oil and the liquid refrigerant, so that after once collected over the height of equalizer pipe 9, the surpluses of them will transfer into the second compressor 2. In this case, since the specific gravity of oil is smaller than that of refrigerant, only the refrigerant is collected over the height of the equalizer pipe 9 in the shell bottom of the first compressor 1. Accordingly, the problems have been occurred that the oil in the first compressor 1 is in short supply causing improper lubrication when starting operation, and that the oil in the second compressor 2 is oversupplied which leads to an increase the throwing out of oil according to a foaming action and overheat by a discharge muffler in the shell and then to an excessive rise of the oil temperature.